Severe combined immunodeficiency (SCID) is a rare, but life-threatening inherited disorder in which infants appear healthy at birth, but lack immunity provided by T and B lymphocytes. Affected infants are unable to resist infections. They develop severe infectious diseases and do not survive unless they receive immune- reconstituting treatment, such as a bone marrow or hematopoietic cell transplant (HCT), or in some cases enzyme replacement therapy or gene therapy. At least 14 known genes can cause SCID, but in at least 10% of cases a gene diagnosis has not been found. Diagnosing SCID soon after birth, before infections develop, offers the best chance for successful treatment. Population based newborn screening (NBS) for SCID has therefore been developed. Dr. Puck helped to pioneer screening for T lymphopenia based on quantitating T cell receptor excision circles (TRECs) in DNA from dried blood spots (DBS). TRECs are present in newly formed T cells, but essentially absent in the blood of infants with SCID, who lack T cells. TREC NBS is now done in CA following pilot trials in WI and MA; several more states are already performing or are planning to perform TREC NBS. Non-SCID conditions with low T cells, including leaky SCID, SCID variants, syndromes with T lymphopenia, and secondary T lymphopenia, are also detected by TREC screening and also confer a risk of infection and immune dysregulation, thus benefitting from early diagnosis. However, questions remain about the incidence and spectrum of SCID and T lymphopenic disorders detected by the TREC test. We will evaluate TREC screening in >500,000 infants born per year in CA (2,500,000 in 5 years), monitoring clinical features, demographics, and associated co-morbidities of infants with low TRECs and T lymphopenia in the newborn period. We will investigate the molecular defects in infants with both typical SCID (defined by very low TRECs, <300 autologous T cells/uL and <10% of normal T cell function) as well as infants with less profound T cell defects. We will prospectively follow infants with non-SCID T lymphopenia found by NBS and also measure newborn TRECs in archived NBS samples from CA-born children with immunodeficiencies. By combining immunophenotyping, deep sequencing and genomic analysis with in vitro studies we will find new disease genes and more fully define the spectrum of human T lymphopenic diseases. These studies will establish the clinical utility of newborn screening for SCID and related T cell disorders while advancing our understanding of human primary immunodeficiencies.